Today, there are many radio and cellular access technologies and standards such as GSM/GPRS, WCDMA/HSPA (Wideband Code Division Multiple Access/High Speed Packet Access), CDMA (Code Division Multiple Access)-based technologies, WiFi (Wireless Fidelity), WiMAX (Worldwide Interoperability for Microwave Access) and recently LTE (Long Term Evolution), to name a few. The technologies and standards have been developed during the last few decades, and it can be expected that the development will continue. Specifications are developed in organizations like 3GPP, 3GPP2 and IEEE. 3GPP is responsible for the development and maintenance of GSM/GPRS, WCDMA/HSPA and LTE standards.
Various frequency bands are typically allocated and/or sold by government organizations, such that an operator may “own” certain bands for a particular use (i.e. the right to use the band in a certain way). Regulations may specify that the owner, i.e. the operator, should deploy a particular technology in a particular frequency band. In some cases, the operator may be able to choose what technology and standard to deploy in their spectrum provided the choices fulfill certain criteria set up by e.g. the ITU (International Telecommunications Union).
As a consequence of the fact that spectrum is a scarce resource, an operator may have the rights to deploy a new cellular access, such as LTE, in a limited spectrum of, say 20 MHz.
However, the fact that the operator may have an existing customer base with existing terminals will prevent the operator from deploying only one technology in the whole spectrum owned by the operator. This could be the case e.g. for an operator that has a large customer base with WCDMA/HSPA subscriptions using the Universal Terrestrial Radio Access Network (UTRAN), and the operator wants to deploy the most recent evolution, the Long Term Evolution (LTE) of UTRAN, also called Evolved-UTRAN (E-UTRAN).
In this example, the operator may then have to divide the available bands between HSPA and LTE. At initial deployment of LTE, the operator may thus continue to use e.g. 10 MHz (corresponding to two WCDMA carriers) with HSPA and reserve 10 MHz for initial LTE deployment.
However, such partitioning of the scarce spectrum to different technologies has some undesired effects on performance:                There is a direct correlation between the peak-rate that can be offered and the spectrum width that is used. Thus, limiting the bandwidth of both HSPA and LTE to 10 MHz in the example above will roughly limit the peak-rate offered to customers to a half. Thus, assuming now, for the sake of illustration, that the technologies can offer around 100 Mbps in 20 MHz, it will mean that the peak-rate will now be limited to around 50 Mbps in each of the technologies.        Initially, it may happen that the HSPA carriers are very loaded, while the LTE carriers in the example only have a few users. Thus, there would be an imbalance between allocation and usage resulting in undesired congestion on the HSPA carriers. However, in order to offer a decent bit-rate on the LTE carriers, it is still not possible to allocate e.g. only 5 MHz to LTE customers, since then LTE would not provide competitive performance in relation to HSPA.        
There have been discussions to find a solution for simultaneous use of multiple radio access technologies (LTE+HSPA carrier aggregation), such that higher peak rates and load balancing can be offered in heterogeneous deployments including at least two radio-access technologies. Both LTE carrier aggregation (CA) as well as HSPA carrier aggregation, i.e. carrier aggregation within the same RAT, is defined in the Release 10 standard of the 3GPP specification. In fact HSPA CA is defined already in Release 9.